Semiconductor devices wherein TFT are established on an insulating substrate such as glass, such as image sensors and active type liquid crystal display apparatus in which TFT are used to drive the picture elements for example, have been developed in recent years.
TFT in which silicon semiconductors are used in the form of a thin film have generally been used for the TFT in such devices. The thin film silicon semiconductors can be broadly classified into two types, namely those consisting of an amorphous silicon semiconductor and those consisting of a silicon semiconductor which has crystallinity. The amorphous silicon semiconductors can be manufactured comparatively easily with a gas phase method, having a low manufacturing temperature, and they are suitable for mass production, and so they are used most generally, but their properties, such as their electrical conductivity for example, are poor when compared with those of the silicon semiconductors which have crystallinity and so, in the future, there will be a considerable demand for the establishment of a method for the manufacture of TFT consisting of silicon semiconductors which have crystallinity in order to attain higher speeds.
The properties of the gate insulating film do not present much of a problem in the case of a TFT where amorphous silicon which has a small mobility has been used. For example, a silicon nitride film which has poor electrical characteristics when compared with silicon oxide can be used for the gate insulating film in a TFT in which amorphous silicon has been used. However, with a TFT in which a crystalline silicon film which has a high mobility is used, the characteristics of the gate insulating film which are about the same as those of the silicon film itself present a major problem.
A thermal oxide film is preferred for the gate insulating film. For example, a gate insulating film can be obtained using the thermal oxidation method if the substrate can withstand high temperatures, being a quartz substrate for example. (For example, JP-B-H3-71793) (The term "JP-B" as used herein signifies an "examined Japanese patent publication".)
A high temperature of at least 950.degree. C. is required to obtain a silicon oxide film which is satisfactory for use as a gate insulating film using the thermal oxidation method. However, there are no other substrates except quartz which can withstand such high temperature treatment, and quartz substrates are expensive and, moreover, there has been a problem in that the production of larger areas has been difficult because the melting point is so high.
However, less expensive glass substrate materials have the problem that their distortion point is less than 750.degree. C., usually 550-650.degree. C., and thus the substrate cannot withstand high temperatures required to obtain a thermal oxide layer using normal methods. Consequently, gate insulating films have been formed using the physical gas phase growing methods (PVD methods, for example the sputter method) and chemical gas phase growing methods (CVD methods, for example the plasma CVD and photo CVD methods) with which they can be formed at lower temperatures.
However, insulating films which have been manufactured using PVD methods or CVD methods have a high concentration of hydrogen and unpaired bonds, for example, and furthermore the boundary surface characteristics are not good. They are therefore also weak in respect of the implantation of hot carriers for example, and centers for charge capture are easily formed, originating from the unpaired bonds and hydrogen. Consequently, when these films are used as gate insulating films for TFT, there is a problem in that the electric field mobility and the sub-threshold characteristic value (S value) are not good, or there are problems in that the gate electrode leakage current increases and the ON current is reduced (deterioration, change with the passage of time).
For example, generally, in those cases where the sputter method which is a PVD method is used, a film of a compound of essentially just oxygen and silicon is formed in principle if a synthetic quartz target comprising oxygen and silicon of high purity is used for the target. However, it is very difficult to obtain a silicon oxide film in which the proportions of oxygen and silicon in the film obtained are close to the stoichiometric ratio and in which there are few unpaired bonds. For example, oxygen is preferred as the sputter gas. However, oxygen has a low atomic weight and so the sputter rate (the accumulation rate) is low, and it is inappropriate as a sputter gas when mass production is being considered.
Furthermore, although a satisfactory rate of film formation can be obtained in an atmosphere of argon, for example, the proportions of oxygen and silicon differ from the stoichiometric ratio and the material obtained is very inappropriate as a gate insulating film.
Moreover, it is difficult to reduce the number of unpaired silicon bonds whatever the sputtering atmosphere, and the unpaired silicon bonds Si. or SiO. must be stabilized as Si--H and Si--OH by carrying out a heat treatment in a hydrogen atmosphere after film formation. However, the Si--H and Si--OH bonds are unstable and they are easily broken by accelerated electrons, and they inevitably revert back into unpaired silicon bonds. The presence of the weak Si--H and Si--OH bonds is the cause of the deterioration which is caused by hot carrier implantation mentioned above.
Similarly, a large amount of water in the form of Si--H and Si--OH is included in a silicon oxide film which has been manufactured using the plasma CVD method, and there are many unpaired bonds, and this also gives rise to the problems mentioned above. In addition, in those cases where tetraethoxysilane (TEOS) has been used as a source of silicon which can be handled comparatively easily, there is a problem in that a high concentration of carbon is included in the silicon oxide film. The present invention provides a means of resolving the problems described above.